According to emedicinehealth.com, 90-95% of men and women have been infected with the Epstein-Barr virus at some point in their lives. Most people don't develop symptoms, so the vast majority of infections do not result in full-blown mono. The virus is contagious through saliva, so it's not surprising that the infection rate among college students has been estimated to be about 15%.
For more information on mono visit the National Center for Infectious Diseases or the Mayo Clinic.